Our lab has a long standing interest in the structural investigation of phage T4 lysozyme. Most of the research focuses on the stability and folding characteristics of this enzyme. With the proposed experiments we would like to investigate accommodations of insertions into loop structures and mutant proteins where beta-sheet regions are replaced by sequences that have a high helix forming propensity. The proposal is in three parts. In the first two, only medium to low resolution datasets could be obtained in house and it is necessary to improve the quality of the diffraction data to be able to understand the structural changes. In the third part we wish to use the synchrotron to obtain, for the first time, a 1 E high resolution structure of the enzyme and to resolve a controversy between the X-ray data and theoretical calculations.